Switchable display apparatus

ABSTRACT

In one aspect, a display apparatus comprises: an emissive spatial light modulator comprising an array of pixels of organic electroluminescent material each arranged to output substantially polarised light; a switchable polariser being switchable between a first and a second polarisation mode in which the polariser passes light of respective polarisation components; and a birefringent lens positioned to receive light from the spatial light modulator. The birefringent lens is arranged to direct light of a first polarisation component into a first directional distribution and to direct light of a second polarisation component into a second directional distribution different from the first directional distribution, the birefringent lens and the switchable polariser being positioned in series. In another aspect, a display apparatus comprises: an emissive spatial light modulator comprising an array of pixels each arranged to output randomly polarised light; a birefringent lens positioned to receive light from the spatial light modulator arranged in a first mode of operation of the display apparatus to direct light of a first polarisation component into a first directional distribution and in a second mode of operation of the display apparatus to direct light of a second polarisation component into a second directional distribution different from the first directional distribution; a quarter waveplate; and a linear polariser. The quarter waveplate is arranged between the spatial light modulator and the birefringent lens and the linear polariser is arranged on the opposite side of the birefringent lens from the quarter waveplate.

The present invention relates to an apparatus for the display of images.

Such an apparatus may be used in an autostereoscopic three dimensionaldisplay, a switchable two dimensional (2D)/three dimensional (3D)autostereoscopic display or a switchable high brightness display system.Such systems may be used in computer monitors, telecommunicationshandsets, digital cameras, laptop and desktop computers, gamesapparatuses, automotive and other mobile display applications as well astelecommunications switching applications.

Display systems which use micro-optic components in order to enhancetheir functionality include Liquid Crystal Display (LCD) projectors,autostereoscopic 3D displays and brightness enhancing reflectivedisplays.

In each system, a micro-optic component such as a microlens array isaligned with at least one pixel of the spatial light modulatorcomponent. A possible system is shown in plan view in FIG. 1. Abacklight 2 produces illumination 4 of an LCD input polariser 6. Thelight passes through a thin film transistor (TFT) substrate 8 and isincident on a pixel layer 10 comprising individually controllable phasemodulating pixels 12-26. The pixels are arranged in rows and columns andcomprise a pixel aperture 28 and a separating black mask 30. The lightthen passes through an LCD counter substrate 32 and a lens carriersubstrate 36 upon which is formed a birefringent microlens array 38. Thebirefringent microlens array 38 comprises an isotropic lensmicrostructure 40 and an aligned birefringent material with an opticalaxis direction 42. The output of the birefringent lens then passesthrough a lens substrate 44 and a polarisation modifying device 46.

Each birefringent lens of the lens array is cylindrical; the lens array38 is a lenticular screen and the geometrical axis of the lenses is outof the page. The pitch of the lenses in this example is arranged to besubstantially twice the pitch of the pixels of the display such that atwo view autostereoscopic display is produced.

In a first mode of operation, the polarisation modifying device 46 isconfigured to transmit light with a polarisation state which is parallelto the ordinary axis of the birefringent material of the microlensarray. The ordinary refractive index of the material (such as a liquidcrystal material) is substantially matched to the index of the isotropicmicrostructure 40. Thus the lenses have no optical effect and there issubstantially no change to the directional distribution of the output ofthe display. In this mode, an observer will see all the pixels 12-26 ofthe display with each eye, and a 2D image will be produced.

In a second mode of operation, the polarisation modifying device 46 isconfigured to transmit light with a polarisation state which is parallelto the extra-ordinary axis of the birefringent microlens array. Theextraordinary refractive index of the material (such as a liquid crystalmaterial) is different to the index of the isotropic microstructure 40.Thus the lenses have an optical effect and there is a change to thedirectional distribution of the output of the display. This directionaldistribution can be set as well known in the art so as an observercorrectly positioned at the front of the display will see a left imagein their left eye corresponding to light from left image pixels12,16,20,24 and in their right eye will see a right image correspondingto right image pixels 14,18,22,26. In this way, a switchable 2D to 3Dautostereoscopic display can be produced.

Lens arrays are particularly suitable for autostereoscopic displaysbecause they combine the functionalities of high efficiency, small spotsize and capability of being manufactured using well known lithographicprocessing techniques.

It has been suggested to provide electrically switchable birefringentlenses for purposes of switching light directionally, for example toswitch a display between a 2D mode of operation and a 3D mode ofoperation.

Electrically switchable birefringent liquid crystal microlenses aredescribed in European Optical Society Topical Meetings Digest Series:13, 15-16 May 1997 L. G. Commander et al “Electrode designs for tuneablemicrolenses” pp48-58.

In another type of switchable 2D-3D display disclosed in U.S. Pat. No.6,069,650 and WO-98/21620, switchable microlenses comprising alenticular screen filled with liquid crystal material are used to changethe optical power of a lenticular screen.

Known organic electroluminescent displays may use reflective electrodesin front of or behind the emissive part of the pixels. The pixelaperture is divided into an emissive region and a gap region, comprisingan emissive pixel aperture. The vertical aperture ratio of the emissiveregion may be limited for example by the required width of the gap inthe row electrode. Electroluminescent displays may also employ activematrix backplanes similar to those used for LCD displays. Again theresult is a reduction in the aperture ratio (emitting area/whole pixelarea). Such panels are thus well suited to brightness enhancement asdescribed in WO-03/015424. In such brightness enhancement, a microlensarray is used to direct an image of the pixel to an optical pupil, or‘window’ in the nominal viewing plane. In the window, the observer willsee an increase of brightness proportional to the vertical apertureratio of the panel. Out of the viewing window, an observer will see thegaps between the pixels, and the display has reduced brightness.

Emissive displays, such as inorganic and organic electroluminescentdisplays including polymer and small molecule organic electroluminescentdisplays typically produce an unpolarised optical output. However,directional distribution optical switching systems may rely onpolarisation switching in order to enable a display to be reconfiguredbetween a first mode, which may be Lambertian for example, and a secondmode, which may be autostereoscopic 3D windows for example. Unpolariseddisplays will thus show a polarisation loss when combined withpolarisation directional distribution optical switching systems.

It is known in the art to use a circular polariser to avoid reflectionsfrom these electrode layers in which the emission is substantiallyrandomly polarised. The circular polariser serves to cancel thereflection of external light from the electrodes, and generallycomprises a linear polariser and quarter waveplate. It would bedesirable to apply such a circular polariser to a display apparatushaving a lens which is capable of modifying the directional distributionof the output light, for example to produce a 3D autostereoscopic effector an enhanced brightness effect. However, in the case that the lens isa birefringent lens which is used to allow switching of the modificationmade by the lens which operates in reliance on the polarisation of lightpassing through the display, it is not self-evident how to implement thecircular polariser which also relies on the polarisation of the light.

In one form according to a first aspect of the present invention, thereis provided a display apparatus comprising:

an emissive spatial light modulator comprising an array of pixels oforganic electroluminescent material each arranged to outputsubstantially polarised light;

a switchable polariser being switchable between a first and a secondpolarisation mode in which the polariser passes light of respectivepolarisation components; and

a birefringent lens positioned to receive light from the spatial lightmodulator arranged to direct light of a first polarisation componentinto a first directional distribution and to direct light of a secondpolarisation component into a second directional distribution differentfrom the first directional distribution, the birefringent lens and theswitchable polariser being positioned in series.

Polarised light from the spatial light modulator passes through theswitchable polariser and the birefringent lens (in either order). Themode of the polariser effectively selects light corresponding to one orother of the first polarisation component or the second polarisationcomponent to be output from the display device. Accordingly, switchingthe mode of the polariser causes the light output from the displaydevice to switch between the first and second directional distributions.

In many practical embodiments, the birefringent lens has substantiallyno effect on light of the second polarisation component so that thesecond directional distribution is the same as the directionaldistribution of the light input to the birefringent lens. This allowsswitching of the device between a mode in which the birefringent lenshas no effect and a mode in which the birefringent lens modifies thedirectional distribution of the display device.

Thus, the first aspect of the present invention provides high opticalefficiency in an emissive displays by aligning the output polarisationof a polarised emissive displays with the input polarisation state ofdirectional distribution optical switching systems. The polarisationalignment may be achieved by means of uniaxial aligned chromophores ofthe emissive material in the emissive pixels of the display. Thealignment direction of the major axis of the polarisation output may beset to cooperate with the alignment directions of the birefringentmaterial in the birefringent lens.

In this way, a high efficiency emissive directional distribution opticalswitching display may be achieved. Such a display has additionaladvantages over LCD displays, for example not requiring a backlight andthus can be made thinner and lighter which can be important for mobileapplications.

In another form according to the first aspect of the present invention,there is provided an optical apparatus comprising:

an emissive display light direction switching apparatus, comprising: anemissive spatial light modulator apparatus comprising an array ofemitting pixel regions each with a substantially polarised opticaloutput;

a switchable polariser being switchable between a first polarisationmode that passes light of a first polarisation component and a secondpolarisation mode that passes light of a second polarisation component;and

a birefringent lens of a birefringence such that in operation thebirefringent lens directs light of the first polarisation componentsubstantially into a first directional distribution and light of thesecond polarisation component substantially into a second directionaldistribution different from the first directional distribution;

the switchable polariser and the birefringent lens being positioned inseries and arranged such that, when input light comprising or resolvableinto both the first and second polarisation components is input to thedevice, light output by the device is substantially of the firstpolarisation component and is substantially directed into the firstdirectional distribution when the switchable polariser is set to thefirst polarisation mode, whereas light output by the device issubstantially of the second polarisation component and is substantiallydirected into the second directional distribution when the polariser isset to the second polarisation mode.

Preferably, one or more of the following optional features is present.

The emissive material may be a polymer electroluminescent material or asmall molecule electroluminescent material.

The substantially polarised output may be:

-   -   a linear polarisation;    -   substantially the same direction for all of the pixels;    -   achieved by uniaxially aligned chromophores; and/or aligned        parallel to the geometric optical axis of the birefringent lens.

A clean-up polariser may be used. It may be positioned between thepixels of the emissive display and the other optical components. Theclean-up polariser may have a transmission axis which is substantiallyparallel to the major axis of the output polarisation of the emissivedisplay.

The display apparatus according to the first aspect of the presentinvention may include any of the features of the display apparatusesdisclosed in WO-03/015424, which is incorporated herein by reference,including all the features in the claims of WO-03/015424, with thechange that the display apparatus has an emissive spatial lightmodulator comprising an array of pixels of organic electroluminescentmaterial each arranged to output substantially polarised light. Theadvantages of the display apparatuses disclosed in WO-03/015424 applyequally to the present invention.

In one form according to a second aspect of the present invention, thereis provided a display apparatus comprising:

an emissive spatial light modulator comprising an array of pixels eacharranged to output substantially randomly polarised light;

a birefringent lens positioned to receive light from the spatial lightmodulator arranged in a first mode of operation of the display apparatusto direct light of a first polarisation component into a firstdirectional distribution and in a second mode of operation of thedisplay apparatus to direct light of a second polarisation componentinto a second directional distribution different from the firstdirectional distribution;

a quarter waveplate; and

a linear polariser,

wherein the quarter waveplate is arranged between the spatial lightmodulator and the birefringent lens and the quarter waveplate isarranged on the opposite side of the birefringent lens from the linearpolariser.

The quarter waveplate and the linear polariser in combination act as acircular polariser to reduce reflections in the known manner referred toabove. This effect is achieved in a display apparatus having abirefringent lens which allows the directional distribution of theoutput light to be modified, with the modification being switchable. Ithas been appreciated by the present inventors that the location of theelements of the circular polariser, namely the quarter waveplate and thelinear polariser, on opposite sides of the birefringent lens stillallows both proper switching of the effect of the birefringent lens andproper operation of the circular polariser in both modes of operation ofthe birefringent lens, notwithstanding that both effects arepolarisation-dependent.

Furthermore the location of these elements achieves significantadvantages as follows. Firstly, as compared to the notional possibilityof arranging the circular polariser as a whole between the spatial lightmodulator and the birefringent lens, only the quarter waveplate islocated there. As a result the distance between the spatial lightmodulator and the birefringent lens may be minimised. This is asignificant advantage. For example in the case of the lens providing anenhanced brightness effect, the viewing freedom of the display isdetermined by the separation of the lens from the pixel plane and thevertical extent of the pixels of the display, so the minimisation of theseparation achieve by the present invention optimises the viewingfreedom of the display in the directional mode. In contrast, minimisingthis distance in a standard 2D display does not improve the viewingfreedom.

Another advantage relates to the losses. An unpolarised emissive displayis cheaper and easier to manufacture than a polarised emissive display.However, when used in combination with an unpolarised emissive dispaly,use of a birefringent lens to provide switchable modification of thedirectional distribution reduces the nominal output by 50% due to theeffect of the polarisation control. Similarly, the linear polariser of acircular polariser, when used with an unpolarised emissive displaywithout a birefringent lens, reduces the normal output by 50%. However,the arrangement of the first aspect of the present invention allows boththe circular polariser and switchable birefringent lens to beimplemented with a total loss of only 50%, ie the losses of the circularpolariser and switchable birefringent lens in isolation do notaccumulate. This is a significant advantage as it allows both featuresto be incorporated without a corresponding increase in the losses.

In another form according to the second aspect of the present invention,there is provided an emissive directional display comprising:

a substantially randomly polarised output emissive spatial lightmodulator comprising an array of pixels;

a passive birefringent lens;

a polarisation rotation apparatus;

a quarter waveplate;

a linear polariser,

wherein the quarter waveplate is positioned between the array of pixelsand the passive birefringent lens.

In a yet another form according to the second aspect of the presentinvention, there is provided an emissive directional display comprising

a substantially randomly polarised output emissive spatial lightmodulator comprising an array of pixels;

-   -   an active birefringent lens;    -   a quarter waveplate;    -   a linear polariser,

wherein the quarter waveplate is positioned between the array of pixelsand the active birefringent lens.

Preferably, one or more of the following optional features is present.

The emissive material may be a polymer electroluminescent material or asmall molecule electroluminescent material.

The geometric optical axis of the lens may be aligned with the pixels ofthe spatial light modulator.

The pixels may be arranged in rows and columns.

The birefringent lens and switching arrangement therefor in accordancewith the second aspect of the present invention may include any of thefeatures of corresponding elements of the display apparatuses disclosedin WO-03/015424, which is incorporated herein by reference, includingthe features in the claims of WO-03/015424.

With both aspects of the invention, the lens array may be used to modifythe directional distribution of the display device, in one or bothmodes, to achieve a variety of different effects including, but notlimited to, the provision of: a 3D autostereoscopic effect; enhancedbrightness; or a multi-user display system.

Thus such devices can be used for:

an autostereoscopic display means which can conveniently provide amoving full colour 3D stereoscopic image which may be viewed by theunaided eye in one mode of operation and a full resolution 2D image in asecond mode of operation;

a switchable, high brightness display system which in a first mode mayexhibit substantially non-directional brightness performance and in asecond mode may exhibit substantially directional brightnessperformance; or

a multi-viewer display means which can conveniently provide one movingfull colour 2D images to one observer and at least a second different 2Dimage to at least a second observer in one mode of operation and a fullresolution 2D image seen by all observers in a second mode of operation.

Advantageously, by applying the enhanced brightness performance to anorganic electroluminescent display the lifetime of the display can beextended. The brightness enhancement may be used to achieve the desiredbrightness level for a reduced electrical drive load of the pixels ofthe display. A reduction in electrical drive load of the pixels can beused to extend the lifetime of the materials used in the display.

The hereinafter described embodiments of the present invention canprovide the following advantages, singly or in combination:

A switchable directional display apparatus using an emissive display canbe configured with high optical efficiency;

The apparatus allows efficient switching between a high resolution, highbrightness 2D mode and a high brightness 3D mode;

The apparatus allows efficient switching between a standard brightness2D mode and an enhanced brightness 2D mode, effectively increasing theoptical aperture of the display;

The apparatus can be manufactured at low cost;

The optical cross talk of the 3D mode can be optimised;

The use of emissive display allows a thin device to be fabricatedwithout the use of a backlight element;

The elements can be manufactured using known techniques;

The display can operate in a wide range of operating environments.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 shows a type of switchable 2D-3D autostereoscopic displayapparatus using a liquid crystal display;

FIG. 2 shows another type of switchable 2D-3D autostereoscopic displayapparatus using a polarised emissive display;

FIG. 3 shows the another type of switchable 2D-3D autostereoscopicdisplay apparatus using a polarised emissive display;

FIG. 4 shows another type of switchable 2D-3D autostereoscopic displayapparatus using a polarised emissive display;

FIG. 5 shows a type of switchable brightness enhanced display apparatususing a polarised emissive display;

FIG. 6 shows an apparatus comprising a quarter waveplate to cancelfrontal reflections from electrodes in a randomly polarised emissive OELdisplay;

FIG. 7 shows a process for OEL panel construction to allow a shortviewing distance while maintaining structural stability;

FIG. 8 shows a display incorporating an active birefringent lens andquarter waveplate; and

FIG. 9 shows the alignment of optical axis of the quarter waveplate inthe embodiment of FIG. 6.

The various embodiments hereinafter described share a number of commonfeatures. For brevity, in respect of the common features commonreference numerals will be used and a description thereof will not berepeated for each embodiment.

FIG. 2 shows a first embodiment of the present invention. An array ofpixels 50 is formed on a display substrate 48 to constitute a spatiallight modulator. The substrate 48 may comprise an array of addressingthin film transistors and electrodes so that each of the pixels may beindependently addressed with an electrical signal. The thin filmtransistors may be inorganic or may be embodied in organic materials.Alternatively, the pixels may be addressed by a passive addressingscheme in which addressing transistors need not be present at thepixels.

Each of the pixels 50 comprises an emissive region in which the emissivematerial comprising chromophores is uniaxially aligned so that thepolarisation of emission is substantially linear and substantially inthe same orientation for the entire pixel. Preferably, each pixel isarranged to have substantially the same polarisation direction.

The emissive material of the pixels 50 may be any organicelectroluminescent material, for example a polymer electroluminescentmaterial or a small molecule electroluminescent material. The emissivematerial may be arranged to produce polarised emission by aligning themolecules of the emitting material using any suitable technique. Forexample, the emissive material may be that disclosed in “PolarizedElectroluminescence from an Anisotropic Nematic Network on a Non-contactPhotoalignment Layer”, A. E. A. Contoret, S. R. Farrar, P. O. Jackson,S. M. Khan, L. May, M. O'Neill, J. E. Nicholls, S. M. Kelly and G. J.Richards, Adv. Mater. 2000, 12, No. 13, July 5 p 971 which demonstratesthat polarisation efficiencies of 11:1 can be achieved in practicalsystems.

A further cover substrate 52 is attached to the pixels. The substrate 52may incorporate barrier layers and contrast enhancement black masklayers.

An optional polariser 54 may be attached to the substrate 52.Alternatively, polariser materials may be incorporated at or near to thepixel plane, on the inner surface of the substrate 52 for example.

For an example, one known polarised organic electroluminescent displayhas a polarisation ratio of 11:1. In combination with a typicalpolariser of polarisation efficiency 45%, the overall throughput fromthe light source will be 82.5%, compared to 45% for an unpolarised lightsource in combination with a clean-up polariser.

An optional substrate 56 is attached to the polariser 54, and has abirefringent microlens formed on its surface. The birefringent microlensincorporates a surface relief structure such as a lenticular surfacerelief structure. The surface relief structure may be formed at theinterface of a birefringent material and an isotropic material.

The birefringent microlens incorporates a birefringent material withalignment direction 42, and an isotropic material 40. The birefringentmicrolens may for example comprise an aligned liquid crystal materialsuch as a nematic liquid crystal sandwiched between homogeneousalignment layers on substrate 56 and on the surface of the isotropicmaterial. Homeotropic alignment layers may also be used.

The birefringent material may be UV cured liquid crystal material suchas a reactive mesogen liquid crystal. The relative alignment of liquidcrystal material on the two surfaces of the birefringent lens may beparallel, anti-parallel, or there may be twist between the two surfacesso that an incident polarisation is rotated in the birefringent lensbefore encountering the surface relief structure. The refractive indexand dispersion of the isotropic material may be substantially the sameas one of the refractive indices and dispersion of the birefringentmaterial. Embodiments of birefringent microlenses which are applicableto the present invention are described in WO-03/015,424, which isincorporated herein by reference.

Following the birefringent microlens, a polarisation switch cell isformed on a substrate 41. The switch serves to switch the polarisationtransmitted through a final linear output polariser 66 and may comprisea layer of nematic liquid crystal material 60 sandwiched betweentransparent ITO electrodes and alignment layer 58. In order to switchthe output polarisation from the switch cell, a voltage 62 is appliedacross the liquid crystal cell.

The apparatus of FIG. 2 operates in the following manner. The emissivedisplay produces a substantially linearly polarised output polarisation.The output polarisation from the polarised emissive pixel array 50 iscleaned by the linear polariser 54 which has a transmission directionparallel to the major axis of the polarisation direction of the emissivematerial. This polarisation state is aligned at 45 degrees to thealignment of the liquid crystal material in the birefringent lens 42, sothat it is resolved in to two orthogonal components by the lens. Thepolarisation switching material 60 in a first state is aligned so thatthe polarisation state transmitted through the output polariser 66 isparallel to the normal refractive index of the liquid crystal materialin the birefringent lens 42. This refractive index is substantiallymatched to the refractive index of the isotropic material 40, and thusthere is substantially no lens effect, although there may be a smallresidual optical effect to the extent that it is not possible to obtaina precise refractive index match. The display then has a directionaldistribution which is substantially the same as the optical output fromthe pixel plane.

In a second mode, the switch 62 adjusts the material 60 so that thetransmitted polarisation through the polariser 66 has seen theextraordinary refractive index of the birefringent material 42 and thusthere is an index step to the isotropic material at the lens surface,and the lens has an optical function. This causes a change in thedirectional distribution of the optical output. The lens may be arrangedto produce an image of the pixel plane at a window plane.

In this specification, the direction of the optical axis of thebirefringent material (the director direction, or the extraordinary axisdirection) will be referred to as the birefringent optical axis. Thisshould not be confused with the optical axis of the lenses which isdefined in the usual way by geometric optics.

The alignment of liquid crystal in the lens may be set to be parallelthrough the thickness of the cell. The alignment at the lens surface maybe parallel to the geometric lens axis of the cylindrical lenses.Alternatively, the birefringent optical axis may be arranged to rotatethrough the cell so that the polarisation alignment direction of theemissive display and clean up polariser are at an angle different to thegeometric lens axis. This may be advantageous for example to relaxmanufacturing tolerances in fabrication of the polarised emissiondevices, or to improve the viewing angle of the devices if there arerestrictions placed on viewing angle by the polarised emissionconditions.

The lenses may be arranged in columns with a pitch substantially (butnot exactly) twice the pitch of the columns of pixels of the display. Ifa user places their eyes in the window plane, alternate columns ofpixels of the panel may be seen, and a stereo image may be observed.This optical output is described in WO-03/015424.

Alternatively, the lens can be arranged as rows of cylindrical lenseswith a pitch substantially the same (but not exactly) as the pitch ofthe rows of the display. If the aperture ratio of the pixels is lessthan 100% in the vertical direction, then in the directional mode ofoperation, the lenses will produce regions from which the display willhave a higher brightness separated by regions of lower brightness due tothe focusing of the lenses from the centre of the pixels.

The apparatus of FIG. 2 has a nominal 50% brightness because the inputpolarisation state to the lenses is resolved in to two orthogonalcomponents respectively parallel and orthogonal to the birefringentoptical axis of the birefringent microlenses.

An apparatus which can show a full brightness mode of operation isillustrated in FIG. 3. Compared to FIG. 2, the position of thepolarisation switch and birefringent microlens array have been reversed.The output polarisation from the polarised emissive display and clean-uppolariser 54 is incident on the polarisation switching mechanism60,58,62. In a first mode, no polarisation switch takes place so thatthe polarisation incident on the birefringent microlens array 42 isparallel to the ordinary component of the refractive index at the lenssurface, and substantially no lens is seen because of the index match tothe isotropic material 40. The light passes through a final substrate 68which may comprise for example anti-reflection films. Thus the light hassubstantially the same directional distribution as the emissive panel.

In the second mode of operation, the polarisation switch rotates thepolarisation from the panel so that it is parallel to the birefringentoptical axis of the birefringent microlens, and the lens has an opticalfunction.

In this configuration, all of the light sees the correct lens axis andthus there are substantially no losses in the system.

Such a configuration has an increased viewing distance because of thethickness of the additional layers. The thickness can be reduced byremoving the substrate 64 and using cured liquid crystal material forthe birefringent lens, as shown in FIG. 4. An optional substrate 70 isformed on the polariser 54 and has ITO and alignment layers 58 formed onits opposite surface. The birefringent microlens and isotropic materialmay be formed on a substrate 68 with an ITO coating 72. The birefringentmicrolens array 42 may be made from a UV cured material, such as areactive mesogen, and an alignment layer 74 may be formed on itssurface. A switchable polarisation modulating material such as a nematicliquid crystal 60 may be sandwiched between the microlens alignmentlayer 42 and the ITO and alignment layer 58. Alternatively, the ITOcoating 72 may be formed on the UV cured birefringent microlens 42surface in combination with the alignment layer 74. A voltage is appliedto the ITO coatings by an electrical source 62.

In this manner, the separation of the lens from the pixel plane may bereduced. This is particularly advantageous for devices with small pixelsizes. The substrate 70 may also be removed, so that the layers 58 areformed on the polariser 54 for example.

FIG. 5 shows a switchable enhanced brightness display apparatus using apolarised emissive display;

As shown in FIG. 5, the polarised emissive pixels in a pixel plane 50may comprise emissive regions 80 and gaps 82 between the emissivepixels. The gap regions may comprise electrodes or addressingtransistors for example. The lenses may be positioned in rows withrespect to the display. And the pitch of the lenses is set to besubstantially the same as the pitch of the rows of the pixels. Theremainder of the elements of the display may be configured and operateas in FIG. 4 for example. In a first mode of operation, the lens isconfigured to have no optical function so that the light from theemissive region is substantially unmodified by the lenses of thecylindrical lens array. In a second mode of operation the lenses areconfigured to have an optical function so that each of the pixels isimaged by a respective lens to a window plane at a nominal distance fromthe display. If an observer places their eye at the image of the pixelsat the window plane then the display will have an increased brightnesscompared to the unmodified display. If the observer places their eye atthe gap between the images, then the display will have a reducedbrightness than the modified display. In this manner, the displaybrightness can advantageously be improved in cases where the apertureratio of the pixel is less than 100% in a first direction.

Further embodiments of the first aspect of the present invention may beformed as the display apparatuses disclosed in WO-03/015424, which isincorporated herein by reference, but replacing the spatial lightmodulators disclosed therein by an emissive spatial light modulatorcomprising an array of pixels of organic electroluminescent materialeach arranged to output substantially polarised light, as describedabove. Therefore the disclosure of WO-03/015424 applies equally to thepresent invention except for the replacement of the spatial lightmodulator.

FIG. 6 shows in cross section a brightness enhancement display apparatushaving an unpolarised display.

The display apparatus comprises an array of pixels 50 formed on adisplay substrate 48 to constitute a spatial light modulator having thesame construction and arrangement as in the emissive display apparatusesdescribed above except that they output substantially randomly polarisedlight. Thus, the emissive material of the pixels 50 may be any organicelectroluminescent material, for example a polymer electroluminescentmaterial or a small molecule electroluminescent material. Alternatively,the array of pixels 50 and substrate 48 may be replaced by any othertype of emissive spatial light modulator comprising an array of pixelseach arranged to output randomly polarised light

The array of pixels 50 has a vertical aperture ratio less than 100% andemits light towards a quarter waveplate 84. As the light issubstantially randomly polarised, first and second resolved linearcomponents of substantially equal intensity are incident on to the lensarray 42. The first resolved linear polarisation component parallel tothe optical axis of the birefringent material sees a phase step at thelens surface, and so the light from a pixel aperture is directed towardsa window at the nominal viewing position. The resolved componentorthogonal to the birefringent material optical axis sees an index matchat the refractive surface, and so substantially no lens function isproduced. The light is then passed through the switchable polarisationrotator 58-62. In the off state, the first resolved linear polarisationstate is rotated and extinguished by the output polariser 66, whereasthe second resolved linear polarisation state is rotated and transmittedthrough the output polariser 66. When an electric field is applied tothe layer 60, the states are unrotated so the first polarisation stateis transmitted and the second state is absorbed. Thus the displayoperates in the conventional mode.

In the first mode of operation, light incident from an external lightsource 86 in front of the display is polarised by the polariser 66, androtated by the rotator 58-62 so that no phase step is seen at the lens42, 40. The light then passes through the waveplate 84 where it isconverted to a circular polarisation state FIG. 9 shows in more detailthe alignment of the optical axis of the quarter waveplate incooperation with the alignment of optical axis of the birefringent lens.For simplicity of operation, the optical path for light from an externallight source 86 is unfolded, and is shown for the directional mode ofthe apparatus of FIG. 6. The incident light from the light 20, source 86has polarisation direction 93 from the polariser 66 with polarisationtransmission direction 92. The light passes through the polarisationswitch (not shown) such that the polarisation state 94 passes throughthe substrate 41. The polarisation state 97 is incident on thebirefringent optical axis 96 of the birefringent lens 42. In thisexample, the alignment of the birefringent lens is anti-parallel so thatthe alignment direction 98 at the substrate 56 is produced and thepolarisation direction 99 in substrate 56 is produced. The optical axisdirection 100 of the quarter waveplate 84 is set 45 degrees to thedirection 98, being the alignment of the birefringent material in thebirefringent lens on the surface closest to the quarter waveplate. Thequarter waveplate produces a substantially circular polarisation state101. The light reflects from the pixel plane 50 with circularpolarisation state 102, and sees the quarter waveplate axis 100 to givea polarisation state output 104. The quarter waveplate thus serves tooutput a polarisation state which is at 90 degrees to the direction 99on the reflected path. This polarisation states 106, 108 are orthogonalto the birefringent optical axis direction 96, 98 at the lens. At thepolarisation switch, the polarisation state is unrotated so thepolarisation sate 110 passes through the substrate 64 and is incident onthe polariser 66 where it is substantially absorbed.

At reflection from the electrodes in the pixel plane 50, a phase shiftoccurs. The light then passes back through the waveplate 84 to produce alinear polarisation orthogonal to the input, so that a phase step isseen at the lens surface. The polarisation state is again rotated by therotator 58-62 and extinguished by the input polariser 66. In theswitched state, the same phase shift occurs at the reflector, so thereflections are again cancelled by the quarter waveplate and polarisercombination. Thus the frontal reflections from the reflectors arecancelled, while maintaining the switching brightness enhancement orautostereoscopic display function.

Such an external polariser embodiment has the advantage that thevisibility of the lenses in external ambient light is reduced. Externallight sources incident on the front of the display pass through theinput polariser, undergo Fresnel reflections at the lens and othersurfaces with phase steps, (for example from reflective coatings such asITO) and then pass back through the output polariser. Therefore, theexternal polariser absorbs a proportion of the light passing in eachdirection, and thus reduces lens reflections, which advantageouslyincreases display contrast.

The viewing freedom of the display in enhanced brightness mode, or thenominal viewing distance in 3D mode is determined by the separation ofthe pixel and lens planes. It is desirable to minimise the thickness ofthe added layers between these two surface. The quarter waveplate 84 canbe a thin waveplate such as a coated, aligned curable liquid crystallayer. One example of such a material is RM257 available from Merck Ltd.which can be UV cured after alignment on a suitable alignment layer. Atypical thickness would be less than two microns for this layer.Multiple layers can increase the spectral efficacy of the quarterwaveplate as well known in the art.

The substrate 56 may be for example a thin glass Microsheet (SchottA.G.), or may be eliminated by the use of a cured liquid crystalmaterial in the lens 42. To ensure structural stability of theencapsulation layer counter substrate 52, the OEL device may beassembled with the lenses in place.

Such an assembly process is described in FIG. 7. A substrate 41 with anITO layer 58 on one side has an isotropic lens structure 40 formed byknown means such a UV casting or embossing on the second surface. Thesurface may be coated by an alignment layer such as polyimide, or mayhave a diffractive alignment layer structure formed on it. Thediffractive layer structure may be formed in the mastering tool forembossing the surface relief structure so that a single embossing stepis required.

FIG. 7 b shows a curable liquid crystal material layer 42 formed on thesurface of the lenses 40. The alignment of the lens surface is fixed bythe alignment layer on the isotropic lens. The alignment of the oppositesurface may be fixed by an alignment layer on a second substrate (notshown), by a diffractive structure on a plane shim, or may be therelaxed alignment state of the liquid crystal material (i.e. noalignment fixed in the material). If a second substrate is used, it maybe removed after solidification of the lens so as to reduce the overallthickness of the device.

FIG. 7 c shows the attachment of the waveplate. This may be a coatedwaveplate such as a curable liquid crystal polymer, or may be alaminated layer. Alternatively, this layer may be attached to thedisplay counter substrate 52.

FIG. 7 d shows the attachment to the display counter substrate 52. Thecomposite counter substrate is then attached to OEL emissive substrateto provide encapsulation as shown in FIG. 7 e. In the case where colourfilters are fitted to the counter substrate, these may be applied to theplane glass 52 or to the assembled composite substrate.

FIG. 7 f shows the final fitting of the switch cell to the assembleddevice.

In a further embodiment of the invention, the polarisation rotator andpassive birefringent lens can be replaced by an active lens as shown incross section in FIG. 8. The emissive pixel plane 50 directs lightthrough the quarter waveplate 84 and on to an active lens comprisingtransparent electrodes 92,94 and liquid crystal layer 88. A surfacerelief lens 90 has a refractive index substantially equal to theordinary refractive index of the liquid crystal 88. In a first mode, nofield is applied to the cell, and the lens is aligned so that there is aphase step at the lens surface, giving a lens function. The lens isarranged to produce the viewing windows. In a second mode, a field isapplied between the electrodes 92,94 so that the liquid crystal material88 realigns, and an index match takes place at the lens surface. Lightincident from an ambient light source sees a cancellation function dueto the combination of the quarter waveplate 84 and polariser 66, asdescribed previously.

1. A display apparatus comprising: an emissive spatial light modulatorcomprising an array of pixels each arranged to output substantiallyrandomly polarised light; a birefringent lens positioned to receivelight from the spatial light modulator arranged in a first mode ofoperation of the display apparatus to direct light of a firstpolarisation component into a first directional distribution and in asecond mode of operation of the display apparatus to direct light of asecond polarisation component into a second directional distributiondifferent from the first directional distribution; a quarter waveplate;and a linear polariser, wherein the quarter waveplate is arrangedbetween the spatial light modulator and the birefringent lens and thelinear polariser is arranged on the opposite side of the birefringentlens from the quarter waveplate.
 2. A display apparatus according toclaim 1, wherein the birefringent lens is a passive birefringent lensarranged to direct light of a first polarisation component into saidfirst directional distribution and to direct light of a secondpolarisation component into said second directional distribution, andthe display apparatus further comprises a switchable polarisationrotator arranged between the birefringent lens and the linear polariser,the switchable polarisation rotator being switchable between a firstmode in which incident light of said first polarisation component isoutput with a polarisation allowing it to be passed by the linearpolariser and a second mode in which incident light of said secondpolarisation component is output with a polarisation allowing it to bepassed by the linear polariser.
 3. A display apparatus according toclaim 1, wherein the birefringent lens is an active birefringent lensswitchable between a first mode in which light having a polarisationallowing it to be passed by the linear polariser is directed into saidfirst directional distribution and a second mode in which light having apolarisation allowing it to be passed by the linear polariser isdirected into said second directional distribution.
 4. A displayapparatus according to claim 1, wherein said pixels of the spatial lightmodulator contain an organic electroluminescent material.
 5. A displayapparatus according to claim 1, wherein the optical axis of thewaveplate is aligned at substantially 45 degrees to the alignment of thebirefringent optical axis birefringent material of the lens at thesurface of the lens closest the waveplate.
 6. A display apparatuscomprising: an emissive spatial light modulator comprising an array ofpixels of organic electroluminescent material each arranged to outputsubstantially polarised light; a switchable polariser being switchablebetween a first and a second polarisation mode in which the polariserpasses light of respective polarisation components; and a birefringentlens positioned to receive light from the spatial light modulatorarranged to direct light of a first polarisation component into a firstdirectional distribution and to direct light of a second polarisationcomponent into a second directional distribution different from thefirst directional distribution, the birefringent lens and the switchablepolariser being positioned in series.
 7. A display apparatus accordingto claim 6, wherein the second directional distribution is the same asthe input distribution, whereby the birefringent lens has substantiallyno optical effect.